1. Field of the Invention
The energy crisis of recent years has stimulated research in the field of alternate and hybrid fuels. One area of particular interest relates to fuels for commercial and agricultural vehicles that are powered by diesel engines. The prospect of farmers becoming self-sufficient in regard to their energy needs has led to investigations of vegetable oils as diesel fuel substitutes. Deterrents to this concept are the generally inferior fuel properties of crude vegetable oils as compared to those of diesel oil. Of particular concern is the inherently high viscosity which causes poor atomization in direct-injected diesel engines. This results in fouling of the injectors and cylinders as well as a buildup of noncombusted fuel in the crankcase causing a thickening of the lubricating oil. This invention relates to a blended vegetable oil fuel which circumvents many of these problems.
2. Description of the Prior Art
One approach to the utilization of vegetable oil as fuel has been to mix it with conventional diesel oil. Insofar as these blends must contain at least two-thirds diesel fuel in order to have acceptable properties, they fall short of meeting the farmer's goal of energy self-sufficiency. Cracking and refining are effective in upgrading vegetable oils, but add considerably to the expense and also negative direct on-the-farm utilization of the harvested product. Likewise, transesterification with a lower alcohol yields a fuel with lower viscosity and acceptable performance properties, but reduces the feasibility of direct use. Moreover, the esters have a solidification temperature of about 4.degree. C., requiring the use of fuel preheaters in colder climates.
The concept of diluting the vegetable oil with lower alcohols, particularly ethanol, is confronted with many of the same difficulties characteristic of diesel fuel-ethanol hybrids. As pointed out by Wrage et al. [Technical Feasibility of Diesohol, ASAE Paper No. 79-1052 (1979)], the most critical problem is phase separation initiated by the presence of trace amounts of water. The water tolerance of blends decreases with decreasing temperature. At 0.degree. C., a water concentration of only 0.05% will cause phase separation. Since this amount can readily be absorbed in the fuel during transport and storage, anhydrous ethanol-oil blends tend to be impractical.
Accordingly, a preponderance of the research efforts on hybrid fuels has been aimed at increasing the water tolerance to not only allow for water absorption, but also to permit the use of aqueous alcohol. It has been reported that when water is properly incorporated into a diesel fuel, it serves as a heat sink, thereby lowering combustion temperatures and reducing NO.sub.x and smoke emissions [G. Gillberg et al., Microemulsions as Diesel Fuels, pp. 221-231 in J. T. Zung (ed.), Evaporation-Combustion of Fuels. Advances in Chemistry Series No. 166, ACS]. This phenomenon is also discussed by N. R. Iammartino [Chem. Eng. 24: 84-88 (Nov. 11, 1974)], D. W. Brownawell et al., U.S. Pat. No. 3,527,581, and E. C. Wenzel et al., U.S. Pat. No. 4,038,698.
The intimate admixture of water and oil in the presence of one or more surfactants results in either a macroemulsion or a microemulsion. Macroemulsions have dispersed particles with diameters in the 200 to 10,000 nm. range and are not stable, eventually separating into two phases. Microemulsions are transparent, optically isotropic, thermodynamically stable colloidal dispersions in which the diameter of the dispersed-phase particles is less than one-fourth the wavelength of visible light. Considerably more surfactant is required to create a microemulsion than a macroemulsion since the volume of the interphase of a microemulsion is an appreciable percentage of the total volume of the dispersed sphere (the core plus the interphase). Microemulsions are generally accepted as micellar systems and may be classified as detergent or detergentless.
In the commonly assigned U.S. Pat. No. 4,451,265, A. W. Schwab discloses stabilizing a hybrid diesel fuel microemulsion having relatively high levels of water and alcohol by means of a two-component surfactant system. One of the components is N,N-dimethylethanolamine and the other is a long-chain fatty acid substance. Commonly assigned U.S. Pat. No. 4,451,267 shows a hybrid diesel fuel microemulsion in which the surfactant is selected from various trialkylamines aand trialkylamine soaps of fatty acid substances. In application Ser. No. 06/423,402, now U.S. Pat. No. 4,526,586 filed by A. W. Schwab and E. H. Pryde, a nonionic hybrid fuel is formulated from 1-butanol as the surfactant. While these formulations are more water tolerant than many predecessor hybrid fuels, the critical solution temperatures are not low enough to permit full-season use in temperate climates.